1. Field of the Invention
The present invention relates to an image forming apparatus having image bearing members and development units for developing latent images formed on the image bearing members.
2. Description of the Related Art
With some of conventional image forming apparatuses based on the electrophotographic process, an image formed on each of a plurality of image-forming photosensitive drums is transferred onto an intermediate transfer belt facing them, or onto a conveyed transfer material, in succession and layered on top of one another. This process is referred to as an in-line method. A contact development method can be employed by such image forming apparatuses. This contact development method performs development with development rollers, as developer bearing members, in rotational contact with the photosensitive drums.
When development is performed by using the contact development method, since the development rollers and photosensitive drums are rotationally driven while they are in contact with each other, the photosensitive drums and development rollers are worn out by friction therebetween. Therefore, leaving contact between a photosensitive drum and a development roller for a longer duration than necessary shortens their lifetime. To address this situation, a configuration for enabling contact and separation between the development roller and the photosensitive drum is discussed in Japanese Patent Application Laid-Open No. 2006-292868.
However, contact and separation between the photosensitive drum and the development roller in the development process involve member attachment error, driving source (motor) control timing, and other variation factors. In consideration of such variation factors, therefore, conventional techniques provide a predetermined amount of margin before and after the contact duration for performing image formation, thus causing the development rollers to securely contact the photosensitive drums during the image forming duration.
Since this margin causes a contact state between the development roller and the photosensitive drum even while image formation is not performed, the lifetime of the development rollers and photosensitive drums may be shortened.